1. Field of the Invention
The present invention relates to ethylene production operations. Specifically, the present invention relates to isolation valves used in a ethylene production operation.
2. Background
Ethylene belongs to a class of compounds known as alkenes, or commonly referred to as olefins. Ethylene is the largest volume petrochemical produced worldwide. It is used almost exclusively as an intermediate in the production of several industrial chemicals. For example, ethylene may be used in one of several forms of polyethylene, as ethylene glycol to make polyester, in the manufacture of vinyl acetate and vinyl chloride, as a building block for linear alpha olefins, and in the production of styrene.
Several commercialized methods are practiced to synthesize olefins. The majority of commercially produced ethylene is produced by thermal cracking petroleum hydrocarbons with steam. The process is commonly referred to as pyrolysis or steam cracking. Steam crackers can produce ethylene from numerous hydrocarbon feeds including natural gas liquids, light petroleum gases, light paraffinic naphthas, and mixtures thereof. In typical ethylene production the feedstock is heated by heat exchange to between 500-600 degrees Celsius. The feedstock is then mixed with steam. The steam mixed feedstock enters a heated tubular reactor, commonly referred to as a radiant tube or radiant coil where it is heated from 500-600 degrees Celsius to between 700 and 900 degrees Celsius for between 0.1 and 0.5 seconds. The hydrocarbons in the feedstock are cracked during this short period of time into smaller molecules including ethylene. The smaller molecules leave the radiant tube and enter a transfer line exchanger where they are cooled to 500-700 degrees Celsius within 0.01 and 0.1 seconds to prevent degradation of the desired products.
As feedstock is heated in the tubular reactors pyrolytic coke is produced. Some of the coke accumulates on the walls of the furnace tubes. Within the furnace tubes, the coke deposits on the wall of the tube and because the coke is a thermal insulator, it prevents efficient heat transfer from the furnace firebox to the reacting gas within the tubes. To compensate for this the furnace is fired harder. Additionally, the effective coil diameter is reduced by increasing deposition of the coke on the tubes inside surface, thus raising the pressure drop across the coil. Consequently, the furnace inlet pressure must be raised. Both effects are detrimental when optimizing ethylene yield.
During the process, less than 100 percent of the feed is converted per pass because of thermodynamic limitations and in order to maximize the yield of the desired olefin product. Separation equipment is used to recover the unreacted feed from the olefin products and byproducts. The unconverted feed is recycled where it is often remixed with fresh feed.
A furnace must be periodically cleaned to remove the accumulated coating of coke. This cleaning is referred to as decoking. In some cases furnace tubes must be cleaned as often as once every ten days. During each decoking cycle a furnace must be isolated from and reconnected to the flow of feedstock, recycled feed stock and steam, and ports that allows lighter fractions produced during cracking to escape from the vessel back to the fractionator. Accordingly, a series of isolation valves must be utilized to control the flow of feed stock, ethylene, steam and other furnace by-products to facilitate decoking of the furnace tubes and to allow unconverted feed stock to be recycled back through the furnaces. However, debris or other matter may build up on the internal components of the valves which then become inoperable. Each decoking cycle and any valve maintenance or repair represents non-productive downtime for the furnace. Accordingly, minimizing costly down-time by installation of robust isolation valves, which allow for long term, repeatable, secure opening and closing, under adverse environmental conditions, of various lines throughout an ethylene production facility is desirable.